Just lit yourself to three strengths for starters and you'll learn -- mainly you'll learn that you can live with three strengths. The subtleties between some of them are pretty hard to describe, meaning that they are often interchangeable, plus in the real world, you have variations in distance between the lamp and the diffusion and the subject, and variations in the lamp itself, that modify the effect also. And you can always add an intermediate frame of diffusion to further soften a light.
My main point is to not worry so much about this, you'll play with different diffusions over the years and start to settle on certain preferences.
There's always a variety of diffusion materials on the grip truck, but honestly, I always use the same few types. LEE 250, 251 & Opal for just about everything 4x4 and smaller. Light Grid and Half Soft Frost for 6x6 and bigger. There's a fair amount of redundancy between all the different types. As David suggests, you should pick a few and work with them until you figure out what you need and don't need.
Mostly I use Opal, 250, and 216 on 4'x4' frames and smaller, and Half Soft Frost and Light Grid Cloth on larger frames. Sometimes I use Muslin, but more for bounces, and sometimes a Silk on larger overheads frames outdoors, though rarely. Occasionally Full Grid Cloth on large frames but I'm more likely to just double-diffuse, like use a large frame of Light Grid Cloth in front and a 216 or 250 intermediate if I need to spread the light a little more to fill the large frame.
Thanks. How are you finding the Depron so far? I've somehow never heard about it until now.
It's not that widely used in film. It's actually a building insulation material, which is also used by RC aircraft modelers. That may be why you've never heard of it.
It's basically a styrofoam type material, available in a variety of thicknesses. The expanded cell structure of it seems to work very well for diffusing light when you're using 2 or 3mm thick sheets. It is quite brittle, and easy to damage, but the quality of soft light from it is very nice.
I also like depron and use it in front of some open face Lowel's (you have to keep it from getting too close to hot lamps as it will melt). It is a bit of a light hog and I see at least a stop of light loss when using it, but it has a very nice diffused quality that I like. I had never seen it used or heard of it until last year. Fortunately I was able to get a few sheets of the 3 mm. I didn't realize they had discontinued selling it. Does anyone know of any other source or if RC will bring it back eventually?